But there are other semantics in use for this problem. If you look at
the semantics used by w32 "GUI shell" (Explorer) it handles the
situation that x2 exist by creating a new directory with a new name.
(Emacs uses that kind of semantic for other things.)

Actually it does different things if you use C-c C-v or drag and drop.
For drag and drop it does not make a new directory.

I would guess that such "GUI shells" on *nix have similar semantic. Or
what do they do?

Nautilus, the Gnome "GUI shell" behaves the same as the W32 one.

That said, it would be a huge surprise to many if copy-directory didn't behave
as cp.